The present invention refers to rheology modifiers and to mixtures of rheology modifiers for modifying the rheological behaviour of coating compositions, to coating compositions containing these rheology modifiers and to substrates coated with these coating compositions. It also refers to processes for the preparation of the rheology modifiers and the coating compositions and to processes for coating the substrates.
Coating compositions for coating a substrate usually include a rheology modifier in order to adapt the rheological behaviour of the coating color (synonymous to coating composition) to the coating device and coating conditions. Traditional coating techniques such as blade coating and roll coating require coating compositions of relatively high “low shear viscosity” (synonymous to Brookfield viscosity) and low “apparent high shear viscosity”. Curtain coating, a relatively recent coating technique, involves adding the coating composition as a falling thin curtain onto the substrate. This method requires coating compositions of relatively low “low shear viscosity”, but high “apparent extensional viscosity” (synonymous to high strechability), as the coating composition curtain should not break especially when the substrate moves at high speed.
Rheology modifiers are usually polymers. Whereas many rheology modifiers, which in a coating composition exhibit high “low shear viscosity” and low “apparent high shear viscosity”, and thus are suitable for traditional coating techniques, are known, rheology modifiers, which as aqueous solution exhibit high “apparent extensional viscosity” and are thus suitable for curtain coating, are rare.
One rheology modifier, that is commercially available and is capable of providing aqueous solutions of high “apparent extensional viscosities”, is STEROCOLL® BL (BASF, acrylamide/acrylic acid copolymers having a Mw of about 10,000,000). The disadvantage of this rheology modifier is that it is only obtainable as an inverse (water-in-oil) emulsion. Inverse emulsions are unstable as the phases can easily separate and prone to freezing, which can cause the formation of undissolved gel particles. Therefore, inverse emulsions require additional equipment such as mixing or heating devices when stored, transported or added into the coating composition and thus are difficult to handle. In addition, inverse emulsions generate oil waste, and thus for environmental reasons demand a special clean-up after use.
It is a first object of the present invention to provide a rheology modifier for coating compositions, which is capable of providing aqueous solutions of high “apparent extensional viscosities”, and which is easy to handle and does not cause environmental problems.
This object is solved by the rheology modifiers discussed herein.
In some coating processes for preparing coated paper, which include blade coating operations, coating compositions of a higher than usual low “apparent high shear viscosity” are required. Coating compositions having a high amount of polyvinyl alcohol as co-binder are known to fulfill this requirement. However, when waste paper coated with these compositions is recycled, a paper stock containing sticky material of light colour, so-called “white pitch”, is obtained resulting in “white pitch” throughout the paper machine and in low-quality paper. As a consequence, the machine has to be stopped and cleaned, and in order to avoid further formation of “white pitch” an increased demand of chemicals, for example fixatives, in the wet end is needed.
In some cases, the formation of “white pitch” is related to the presence of polyvinyl alcohol in the paper stock as discussed in Teirfolk, J.-E, Hossenlopp, C., Castaing, J.-C., “Interactions between latex, calcium ions and polyvinyl alcohol”, PTS-Streicherei-Symposium, 2001.
Therefore, it is a second object of the present invention to provide a rheology modifier for coating compositions, which is capable of forming coating compositions of higher than usual low “apparent high shear viscosity” without requiring the presence of high amounts of polyvinyl alcohol as co-binder.
This object is solved by the mixture of rheology modifiers described herein.
The rheology modifiers of the first aspect of present invention are water-soluble polymers having a weight average molecular weight (Mw) of at least 1,000,000 g/mol and an intrinsic viscosity of at least 2.5 dl/g, both as determined by size exclusion chromatography, and the rheology modifiers are in the form of solid particles.
Preferably, the polymers have a weight average molecular weight (Mw) of at least 2,000,000 g/mol and an intrinsic viscosity of at least 3.5 dl/g, both as determined by size exclusion chromatography. More preferably, the polymers have a weight average molecular weight (Mw) of at least 2,500,000 g/mol and an intrinsic viscosity of at least 4.5 dl/g, both as determined by size exclusion chromatography.